Digital waveform generators which take an input signal from a source and generate a sinusoidal output are known. One such digital harmonic generator is described in U.S. Pat. No. 4,950,966 to Moulds, III. U.S. Pat. No. 4,878,188 to Zeigler, Jr. also describes an all digital implementation for generating sine and cosine waves.
Vibration isolation and sound isolation systems are also well known in the art. These systems utilize microprocessors to supply canceling waves to control or minimize vibration or sound within a defined area responsive to an external input signal. Examples of such systems are taught in U.S. Pat. Nos. 4,677,676 to Eriksson, 4,153,815 to Chaplin, 4,562,589 to Warnaka et al., 5,170,433 to Elliott, and 4,689,821 to Salikudden et al. which are hereby incorporated by reference herein. In these systems, the control scheme that is used can be least mean square (LMS), Filtered-X LMS, or the like.
In active control systems of the above-mentioned type, it is required to have an input signal to the microprocessor which is indicative of the frequency content of the input source. Particularly, it is required to have at least one analog wave that is synchronized with the source input signal for providing the appropriate information to the processor. All digital implementations for providing-the sine and cosine waves are available and utilize lookup tables. However, all digital implementations add additional processing load to the microprocessor. Thus, they take away from the speed of the processing. As a result, the processor has less time to spend calculating and adapting the filter weights and otherwise attempting to minimize the noise and/or vibration.
U.S. Pat. Nos. 4,551,683 to Matsuo et al., 5,121,069 to Burns et al., and 4,633,425 to Senderowicz describe the use of switched-capacitor filters and are hereby incorporated by reference herein. U.S. Pat. No. 4,027,264 to Gutleber provides an interference canceling system which utilizes a phase-locked loop and is hereby incorporated by reference herein. However, none of the prior art systems provide a waveform generator for providing an analog wave which is frequency insensitive and which does not add additional computational load to the microprocessor.